Firefox 3.5.1 is the first update to the new 3.5 series of Firefox. According to the users' changelog, this release fixes several issues, including security and stability-related problems. To download the new Firefox 3.5.1, go to the Mozilla downloads page. I wrote two guides on installing Firefox 3.5 (and later) in Ubuntu, you can find them here and here. You can watch a video of Firefox 3.5.1 running in Ubuntu below: